Okay, time to serious. Why didn't Octavia dodge? To be honest, I hate when on a TV show or movie, someone is either about to get shot, blown up, sliced by a sword, or get crushed, they don't. Fucking. Dodge. They just wait for someone else to save their ass, and the person that saved that other person ends up being the person that either dies or gets seriously hurt, when all you gotta do, is get out the way, and instead of someone being hurt, everyone is fine.
It is a matter of shock. When somebody is in a ton of danger in a split second in the line of fire, the victim gets in a state of paralysis. People aren't ninjas who react unless they are trained for it just on time. Vinyl was able to react since she saw what was happening and was not in the line of fire at the time, so she was able to save Octy.